The crystal chemistry of L-Ta2O5 and related structures

Single crystals of a new form of L-Ta2O5 with a 19 x b superstructure have been synthesised by flux growth. The phase is most likely stabilised by the incorporation of a small amount of lithium (0.14 wt% Li) from the flux. The phase has C-centred monoclinic symmetry with a 6.1939(4) angstrom, b = 69.549(5) angstrom, (= 19 x 3.66 angstrom), c = 3.8895(3) angstrom, gamma = 90.00(1)degrees. The structure was refined in space group C112/m to R-1 = 0.044 for 814 unique reflections with F>4 sigma(F). The structure can be described as comprising chains of edge-shared TaO7 pentagonal bipyramids that are regularly folded at (010) planes to give sinusoidal chains along [010]. These chains are interconnected along [100] and [001] by corner sharing, creating inter-chain regions that are occupied by isolated TaO6 octahedra and pairs of corner-shared octahedra. A comparison with published data for high-quality refinements of related structures has led to the development of a general model that can explain the structural chemistry variations in the known L-Ta2O5-related structures. A shorthand notation is presented for representing the structures, based on the sequence along [010] of the interchain octahedra. Crown Copyright (c) 2005 Published by Elsevier Inc. All rights reserved.